1. Field of the Invention
This invention relates to non-vertically hinged doors and associated counterbalancing mechanisms. More particularly, it relates to counterbalancing mechanisms which substantially exactly counterbalance the weight of the door at all angles throughout its arc of motion.
2. Description of Related Art
There are numerous applications in which a door is mounted with a non-vertical hinge line and requires counterbalancing. Such applications include, among others, hatch covers for roof openings, flush mounted sidewalk doors, and exterior basement entrance doors.
Doors for use in these applications are often made of metal, for strength and durability and, accordingly, can be quite heavy. Counterbalancing allows the door to be opened and closed more easily, and improves safety by reducing the tendency of the door to close rapidly and with great force when released.
Typically, partial counterbalancing has been derived from one or more torque rods, springs, gas cylinders or weights. Torque rods have been particularly widely used because they provide a counterbalancing torque as a result of the rotation of one end of the rod relative to the other. Thus, the opposite ends of the torque rod may be connected to the door and the door frame, respectively, to provide a simple, but reliable, counterbalancing mechanism. Through appropriate selection of the torque rod diameter and length, a variety of doors of different weights and sizes may be approximately counterbalanced with this direct connection method.
Torque rods also have the advantage that their long, thin shape can be positioned out of the way behind, or within the thickness of the door frame, producing a door assembly that takes the minimum space when held in inventory and is easy to transport through the distribution chain. Moreover, torque rods are extremely rugged and reliable, an important consideration in doors which are often used for exterior access or in exposed locations.
However, a torque rod counterbalancing design using the simple direct connection between door and frame does not provide perfect counterbalancing. This is because a torque rod provides a counterbalancing torque which, in the usual operating range, is linearly proportional to the amount of rotation or twist applied to it. In contrast, a non-vertically hinged door requires a counterbalancing torque which is non-linearly related to the opening angle of the door. The weight of the door unsupported by a non-vertical hinge line increases as a sinusoidal function of the opening angle.
As a result, counterbalancing systems using torque rods directly connected between the door and the frame only provide exact counterbalancing for the door at two different opening angles of the door. These angles may be found on a graph of torque (measured at the hinge line) versus opening angle (zero degrees equals closed) where the line of torque rod generated counterbalancing torque intersects the cosine curve of the torque due to the unsupported weight of the door. For a horizontally hinged door, the entire weight of the door is unsupported by the hinge when the door is just being opened, and all the weight is supported by the hinge as it reaches the fully open, ninety degree, position.
While the specific two opening angles where the door is exactly counterbalanced in a linear counterbalancing system are under the designer's control, they have usually been selected to be at approximately the fully open and fully closed positions. At the fully open position no counterbalancing torque is required, and the torque rod is not twisted. At the fully closed position, the torque rod is adjusted to provide the exact counterbalance torque required for the full weight of the door. Unfortunately, except at these two angles, the door is insufficiently counterbalanced and may begin to move if released.
In addressing this deficiency, subsequent designs for counterbalancing systems have used a cam system with single or multiple torque rods to nearly exactly counterbalance the door throughout its range of motion. In these designs, the torque rod is not directly connected between the door and the frame, but instead acts through a cam which modifies the linear torque produced by the torque rod to match the sinusoidal torque needed to balance the weight of the door. Doors with counterbalancing mechanisms of the cam-based type are seen in U.S. Pat. Nos. 4,873,791 and 5,136,811.
In such cam-based designs, the entire counterbalancing force for the door is applied through the cam mechanism. The present invention, however, uses a hybrid direct connection/cam based design. A portion of the counterbalancing torque is produced by one or more torque rods directly connected between the door and the door frame, and the remainder of the counterbalancing torque (the "augmenting" torque), as needed to provide nearly exact sinusoidal counterbalancing, is applied by one or more additional torque rods through a cam system.
By applying the majority of the counterbalancing torque with the directly connected torque rod, less force passes through the cam system, which reduces friction and wear as compared to earlier designs. Another advantage lies in the flexibility of the choices available in a hybrid direct connection/cam based torque rod design. Because torque rods are often available only in standard diameters, it may be difficult to match the counterbalance requirements of some doors. With the hybrid design, differently sized torque rods may be combined to optimally match the counterbalancing requirements of a wide variety of doors.
Yet another advantage lies in the fact that the augmenting torque rod counterbalancing system may be provided as a factory installed option to a door also sold with only direct torque rod counterbalancing, or it may be used as an add on field installed accessory for an existing directly counterbalanced door.
Bearing in mind the above, it is therefore an object of the present invention to provide a new and improved counterbalanced door and counterbalancing assembly in which the counterbalancing is performed in part by a counterbalancing mechanism producing linear counterbalancing and in part by an augmenting counterbalancing mechanism to counterbalance the door throughout its arc of motion.